This invention relates to a power feed for feeding electrical power into a submarine system, e.g. a submarine power cable for supplying power to repeaters on a submarine fibre optic. It is envisaged that it will be particularly useful in dual-end power feeding systems in which the submarine system traverses a body of water (e.g. the Atlantic) between two countries e.g. the UK and the US, and a power feed is provided in each country such that the submarine system receives power from both ends.
In a conventional prior art dual-end power feeding submarine system, a submarine cable traverses a body of water between two land based power feeds. The cable is run so that it has a potential difference of 10 kV between its ends. It receives a potential of +5 kV at its first end and −5 kV at its second end from the respective power feeds. The cable is at zero potential (virtual earth) at a point near the middle. The electrical power is used to drive repeaters which are spaced along the cable and which are used to regenerate signals in a fibre optic cable running generally inside of the cable.
The power feed includes a controller and electrical power converters. The power converters receive electrical power from a power source in the country in which the power feed is located, usually this will be 50 VDC Telecom derived from A.C. electricity from the countrys' national grid or possibly from a diesel generator. Each converter converts the electrical power which it receives from the local power source into a regulated D.C. signal which it feeds into the power cable.
The characteristics of the D.C. signal output by each power converter, and in particular the output D.C. voltage, is determined by the controller which is connected to each of the power converters. The controller outputs the same control signal to each of the power converters; generally the power converters are controlled by pulse width modulation and the controller is a pulse width modulator.
Generally each power converter operates at maximum efficiency when it is outputting at its maximum output power. In a typical example each power converter is able to output between 0 and 3 kW. The output voltage depends upon the pulse width modulation signal which the power converter receives.
Under ordinary conditions half of the requisite power for operating the submarine system is provided by the first power feed and half of the requisite power is provided by the second power feed. The split need not be exactly 50:50, for example the first power feed could provide 60% and the second power feed 40% of the required power. However in the above example under ordinary conditions 5 kV at 1 A (i.e. 5 kW) is provided by each power feed. This means that under fault conditions when one of the power feeds is out of operation, the remaining power feed must provide all of the requisite power for running the submarine system, i.e. double the power which it supplies under normal operating conditions. Thus each power converter in the power feed is operated at half power providing an output voltage of 1.25 kV under normal conditions providing a total output voltage of 5 kV. Under fault conditions each power converter is run at 2.5 kW (the maximum power) providing a total output voltage of 10 kV at 1 A.
A problem with the above described conventional power feed is that under normal operating conditions each power converter is operated at partial load which is less than the maximum operating efficiency. This wastes energy and also causes the converters to run at a higher temperature than necessary. An object of the current invention is to ameliorate this problem and provide a power feed which operates at greater efficiency.